Optical communication networks employ optical fibers to carry optical data signals. To avoid bottlenecks created by optical-to-electronic and electronic-to-optical conversions at amplifiers, routers, switches, etc., many modern optical communication networks employ end-to-end all-optical connections. Optical splitters divide optical signals from one optical fiber onto multiple optical fibers, which extend in various directions to implement branches of a network. Similarly, optical combiners combine optical signals from multiple optical fibers of various network branches onto single optical fibers. Optical amplifiers compensate for losses experienced by the optical signals traversing the optical fibers, splitter, combiners, etc. In a long-haul network, such as a nation-wide network, an end-to-end connection may include 40 or more optical fibers in tandem and a corresponding number of optical amplifiers.
Erbium-doped fiber amplifiers (EDFAs) are commonly used in optical networks. Many in-band wavelengths of light, each wavelength carrying separate traffic, can be multiplexed and transmitted together over a single optical fiber. Pumped by an out-of-band laser, an EDFA amplifies all in-band wavelengths. Consequently, traffic from many users can be wavelength multiplexed, amplified and simultaneously carried over an optical network as the signal is combined and/or split, as described above. In addition, optical cross-connects (OXCs) and wavelength-selective switches may be used to route traffic through the optical network.
“Optical flow switching” (OFS) is a network architecture that provides end-to-end all-optical connections to users, typically with very large data transactions. Light paths for data flows are scheduled into (possibly future) time slots. However, even using a supercomputer, a prior art scheduler for a wide area mesh optical network with full switchability takes about 12 minutes to compute an assignment for one data transfer request. In addition, to achieve an end-to-end all-optical path, complex wavelength shifting schemes are employed along the path.
Furthermore, in many cases, only a subset of possible wavelengths is illuminated in a given optical fiber, such as to conserve energy or to avoid unnecessary heat generation. If a scheduler cannot schedule a light path between a desired source node and a desired destination node using already-illuminated wavelengths, illuminating an additional wavelength of light in one or more optical fibers may add sufficient bandwidth to accommodate a data transfer request. However, illuminating the wavelength may detrimentally affect traffic being carried by already-illuminated wavelengths in the same branch or in other branches of the network. Similarly, extinguishing an illuminated wavelength may detrimentally affect traffic being carried by other wavelengths in the same branch or other branches of the network.
When a wavelength of light is switched on and off in a meshed network, existing channels (illuminated wavelengths) in the same fiber experience two types of impairments: fast transients and steady-state channel quality variations. Some of these impairments result from a combination of causes, including randomness in EDFA gain, accumulation of amplified spontaneous noise and issues caused by constant-gain control circuits in EDFAs. Cross-channel power coupling may also cause impairments.
EDFAs employ feedback circuits to maintain constant gain or constant power output. However, response times of these feedback circuits are on the order of about 1 ms or longer. As noted, an EDFA amplifies all in-band wavelengths of light. Thus, a sudden change in power to an input of an amplifier, such as due to a channel (wavelength) being added or dropped, causes a large transient of up to several dB in all channels of the amplifier's output, until the feedback circuit restores nominal operation. This transient may cause one or more of the in-use channels to be over-amplified or under-amplified and, therefore, to become out-of-specification downstream, such as at inputs of subsequent amplifiers or optical receivers. At typical optical network speeds, at least tens of millions of data symbols are transmitted in 1 ms. Thus, during the time taken by the feedback circuit to restore nominal operation, much data can be lost.
Prior art methods for adding or deleting a channel (illuminating or extinguishing a wavelength) in an optical communication network involve a time-consuming manual process of gradually adding or deleting the channel, one hop at a time, ramping up or down optical signal levels and manually checking for unacceptable impairment of existing channels in the hop and in other hops that are optically connected to the hop. Currently, this process takes about 17 minutes to add or delete a channel for a coast-to-coast connection. Consequently, in the prior art, once set up, channels are usually left in place for days and handle multiple transactions. Clearly, a faster and more efficient mechanism for adding and deleting wavelengths (channels) is desirable.